Sediment Gravity Flow Deposits Research Articles

Subaqueous sediment gravity flow deposits: practical criteria for their field description and classification

ABSTRACTA new method for the description and classification of subaqueous sediment gravity flow deposits is proposed. The classification scheme employs a convenient letter code and divides deposits (individual beds) into descriptive categories of two hierarchical levels: facies and subfacies. Facies, as the higher rank categories, are distinguished chiefly on the basis of sediment type (i.e. bed grain size/texture). A total of 13 facies have been distinguished: G= gravel; GS = gravel‐sand couplet; GyS = gravelly sand; S = sand; SM = sand—mud couplet; MS = mud—sand couplet; TM = silt—mud couplet; MT = mud—silt couplet; M = mud; MyS = muddy sand; SyM = sandy mud; MyG = muddy gravel; GyM = gravelly mud. Subfacies, as the lower rank categories, are distinguished within the individual facies on the basis of the bed's internal structures. The number of subfacies is unlimited, and their labelling code includes particular facies symbols (see above) preceded by lower—case letters denoting specific sedimentary structures and their vertical arrangement. Subfacies thus refer to the bed's intervals, or divisions, which are labelled as follows: m = massive (unstratified and ungraded); g = graded (unstratified and graded); s = plane‐stratified; x=cross‐stratified; 1 = parallel– and/or cross‐laminated; q = liquefied. For example, subfacies gsG (graded to plane‐stratified gravel) are gravel beds that have a lower graded interval and an upper plane‐stratified interval; subfacies xG (cross‐stratified gravel) are gravel beds that are cross‐stratified throughout; subfacies slS (plane‐stratified to laminated sand) are sand beds that have a lower plane‐stratified interval and an upper laminated (parallel‐ and/or cross‐laminated) interval.

余目油田におけるレザバーキャラクタライゼイション（その１）

A study has been performed to delineate the spatial distribution of the reservoirs and to infer reservoir rock properties in the Amarume oil field. Reservoir rocks consist of lenticular sandstones which are considered to be sediment gravity flow deposits of Neogene. Productive sandstone reservoirs lie approximately 900m below the surface and vary from 5m to 20m in thickness. 3-D seismic data was processed carefully and the filter operator which matches the seismic data to VSP data was applied to time migrated 3-D data volume to compensate for the phase shift. The reliable information of acoustic impedance was extracted from 3-D seismic data by lithologic inversion. It provided us the spatial distribution of thin sandstone reservoirs. The lateral variation of acoustic impedance observed in each sand reservoir suggests that the reservoir rock properties such as porosity change laterally. P-wave and S-wave velocity tomograms between two wells were derived from cross-well seismic data by traveltime inversion and Vp/Vs ratio profile was estimated. The resultant P-wave velocity tomogram was generally consistent with the well log data. Its resolution was less than that of the velocity profile derived from 3-D seismic data. However, cross-well seismic data provided both P-wave and S-wave velocity information which can be powerful tools to infer the reservoir rock properties.

Sediment gravity flow deposits of an Ordovician deep-sea fan system (western Precordillera, Argentina)—reply

Abstract Ordovician siliciclastic sequences of Portezuelo del Tontal Formation (PTF) in the western Precordillera, Argentina, are interpreted to be deep-sea deposits which formed on a passive continental margin. Several facies and layer divisions have been identified and interpreted as high- and low-density turbidites, debrites, and contourites-hemipelagites. Outer and inner (upper-middle) submarine fan subenvironments have been recognized. Outer-fan associations consist of lobe and lobe-fringe sequences. Inner-fan associations are defined by coarse-grained and channelized deposits. The vertical arrangement of facies associations is interpreted in terms of progradational and retrogradational processes. Some sharp progradational and retrogradational transitions were controlled by allocyclic factors, such as sea-level changes and tectonics of the source areas. The submarine fan model for the PTF is an intermediate type between the pelite-rich and the sand-rich fans. It was formed in a narrow and elongate deep-sea basin controlled by the contemporaneous evolution of an oceanic rift in the direction of progradation.

Updoming, rifting and continental margin development during the Late Palaeozoic in northern Oman

Abstract Subsurface data from northern interior Oman and new surface data from the Oman Mountains imply that the north Oman passive continental margin developed in the Permian. Late Carboniferous and Permian updoming of an area roughly overlapping the present Oman Mountains is indicated by the northward thinning of the continental clastics of the Haushi Group (Late Carboniferous and Early Permian) in the sub-surface. Well correlations show the thinning as cutouts at internal unconformities proving contemporary uplift along a roughly E-W strike. Deep water sedimentation off northern Oman commenced already in the early Late Permian. Permian deep water sediments of the Hawasina nappes consist of reef-derived sediment gravity flow deposits, cephalopod limestones and radiolarites that were deposited in a slope and basin floor environment north of a coralgal reef tract. Mafic volcanics underlying these deposits show locally tholeiitic characters suggesting that incipient drifting of the Hawasina ocean occurred as early as the Late Permian. Penecontemporaneous flooding of the Arabian shield occurred on an almost unfaulted substratum even in areas close to the inferred shelf edge. The interpretation of these data in terms of models for continental rifting suggests that north Oman developed on the little faulted side of an asymmetrical rift produced by Permo-Carboniferous low-angle normal faulting of the lithosphere.

Tectonic Significance of Large-Scale Chaotic Deposits in a Cretaceous Fore-Arc Basin: Valle Formation, Cedros Island (Mexico): ABSTRACT

A mappable, deep-marine slide deposit (olistostrome) within medial-Cretaceous fore-arc basin strata (Valle Formation), located on Cedros Island, Baja California Norte, records the initiation of intrabasinal faulting. Studies of both modern and ancient olistostromes show that olistostromes can form in all physiographic provinces (including shelf and abyssal plain) and tectonic settings of the marine environment. A variety of triggering mechanisms have been suggested for olistostromes, including tectonism, sea level changes, diapirism, rapid sedimentation that overloads steep slopes, migration of gas hydrates, or combinations of the above. The olistostrome in the Valle Formation ranges in thickness from 0 to at least 180 m, and extends areally for at least 34 km{sup 2}. It can be divided into two parts. The basal 30-40 m contains large (up to 8 m) angular blocks (allolistoliths) derived from the Jurassic substrate. The allolistoliths decrease in abundance upsection, whereas internally coherent intraformational slide blocks (endolistoliths), which reach tens of meters in width, increase. Beds composing the endolistoliths are alternating mudstone and sandstone turbidites that were deposited on a tectonically stable basin plain or rise setting before catastrophic failure of the sedimentary pile produced the olistostrome. Intrabasinal faulting is invoked as a cause of the sediment failure becausemore » of the presence of allolistoliths, which must have been shed into the basin from uplifted( ) basin floor scarps. Allolistoliths occur sporadically throughout at least 400 m of coarse-clastic sediment gravity-flow deposits that cap the olistostrome, suggesting that intrabasinal faulting continued to affect the basin long after the olistrostrome formed.« less

Sedimentology of the lower huronian supergroup (early proterozoic), Elliot lake area, Ontario, Canada

The Early Proterozoic Huronian Supergroup is a slightly metamorphosed sedimentary succession locally > 10 km thick, exposed to the north of Lake Huron, Ontario, Canada. The lower part of the supergroup, comprising the Matinenda, McKim, Ramsay Lake and Pecors formations, was studied in the Quirke Syncline area, Elliot Lake, and regions immediately to the west and south. Thirteen lithofacies are present: trough and planar cross-stratified sandstone, quartz pebble conglomerate, clay-rich sandstone, lenticular-bedded sandstone-siltstone-mudstone, laminated sandstone-siltstone-mudstone, massive sandstone- massive pebbly sandstone, massive diamictite, stratified matrix-supported diamictite, stratified clast-supported diamictite, graded diamictite, and massive siltstone. The lithofacies may be grouped into associations which correspond approximately to the mapped formations. The crossbedded sandstones and conglomerates comprise a fluvial association (Matinenda Formation), which was formed mainly in shallow braided channels. Clay-rich sandstone and the lenticular and laminated lithofacies, constituting portions of the McKim and Pecors Formations, were deposited on the strand and offshore areas of braid deltas, interfingering laterally and vertically with the Matinenda fluvial deposits. Four glacigenic subassociations are present in the Ramsay Lake Formation. An ice-proximal subassociation (I) consists of massive pebbly sandstone and diamictite which progressively truncates the Matinenda and McKim Formations to the north. The massive sandstone may be an ice-override deposit formed by erosion and reworking of the underlying Matinenda Formation, or a sub-ice shelf rain-out accumulation of locally eroded glacial debris. Subassociations II and III consist mainly of subaqueous sediment gravity flow and ice rain-out deposits. Subassociation IV is interpreted as an ice-proximal fluvial outwash deposit containing resedimented blocks of moraine material. Stratigraphic and paleocurrent data indicate a south-dipping paleoslope that tilted southeast during the deposition of the fluvial and deltaic units. A trough of relatively deep water extended NW-SE across the region, transverse to the E-W divergent continental margin hinge line that is assumed to have existed to the south of the project area. Analogies are suggested to Mesozoic-Recent basins on the Atlantic margin of Canada, where transverse paleogeographic trends reflect either landward extension of active oceanic fracture zones, or reactivation of earlier structural lineaments.

Glaciomarine facies within subglacial tunnel valleys: the sedimentary record of glacioisostatic downwarping in the Irish Sea Basin

ABSTRACTCoastal exposures of Late Pleistocene sediments deposited after 19 000 yr BP near Dublin, Ireland, provide a window into the infill of a subglacially‐cut tunnel valley. Exposures close to the steeply dipping bedrock wall of the valley show boulder gravels within multi‐storey U‐shaped channels cut and filled by subglacial meltwaters driven by a high hydrostatic head. Gravels are truncated by poorly sorted ice‐proximal glaciomarine sediments that record the pumping of large volumes of subglacial debris along the tunnel valley to a tidewater ice sheet margin. The sedimentary succession is dominated by sediment gravity flow facies comprising interbedded diamict and massive, poorly sorted gravel facies interpreted as subaqueous debris flow deposits. Gravel beds show local inverse and normal coarse‐tail graded facies recording the restricted development of turbulent flow. Sediment gravity flow deposits fill broad (<2 km) shallow (10 m) and overlapping channels. Penetrative deformation structures (e.g. dykes) are common at the base of channels.The same subglacially‐eroded topography and glaciomarine infill stratigraphy can be identified on high resolution seismic profiles across nearly 600 km2 of the western Irish Sea. Tunnel valleys are argued to have been exposed to glaciomarine processes by the rapid retreat of a calving tidewater ice sheet margin in response to marine flooding caused by glacio‐isostatic downwarping below the last British Ice Sheet. The facies associations described in this paper comprise an event stratigraphy that may be found on other glaciated continental shelves.

Controls on sedimentation in a late Devensian ice‐dammed lake, Achnasheen, Scotland

Two distinct glacigenic sediment‐landform associations are described from the site of a former ice‐dammed lake at Achnasheen, Ross‐shire, Scotland. Firstly, three cross‐valley ridges, principally composed of rain‐out and sediment gravity flow deposits, were associated with a damming lobe of ice which directly controlled lake level. Secondly, large coalescing meltwater‐fed deltas were developed in front of two outlet glaciers which initially terminated in the western end of the lake. The divergent evolution of the two associations is explained with reference to differing relationships between ice thickness and lake level, flux of meltwater, and glacier activity.

Stratigraphy and sedimentology of the Hamill Group in the northern Selkirk Mountains, British Columbia: evidence for latest Proterozoic – Early Cambrian extensional tectonism

Three informal stratigraphic divisions are recognized in the uppermost Proterozoic – Lower Cambrian Hamill Group in the northern Selkirk Mountains of British Columbia. These informal divisions include a lower sandstone unit, a greenstone–graded-sandstone unit, and an upper sandstone unit. Both the lower and upper sandstone units display sedimentary characteristics that are uniform along strike and indicate a shallow-marine environment of deposition. As is typical of other exposures of the Hamil Group in southeastern British Columbia, the lower sandstone unit is coarser grained and more poorly sorted than the mature quartz arenites of the upper sandstone unit.The greenstone–graded-sandstone unit is a complex assemblage of mafic metavolcanic rocks and associated sandstone facies. This unit is highly variable along strike but essentially consists of a thick succession of subaqueous extrusive rocks overlain by a variety of sediment gravity-flow deposits. These latter deposits include resedimented conglomerates, debris-flow deposits, and trubidites (deposited from both high- and low-density turbidity currents). Stratigraphic sections of this unit are described in detail from three different localities and are examined in terms of their transport and depositional mechanisms.The stratigraphic succession of the Hamill Group indicates that deposition of the shallow-marine sands of the lower sandstone unit was abruptly interrupted by a period of volcanism, the creation of a paleoslope, and the deposition of a large volume of sediment gravity-flow deposits of the greenstone–graded-sandstone unit. These relations are attributed to an episode of syndepositional normal faulting. The inferred fault(s) could have served as the conduit for the extrusion of the volcanics. Offset along the fault(s), the tilting of fault blocks, and the consequent formation of an unstable slope adjacent to a fault scarp created an environment favorable for deposition of the sediment gravity flows. In general, deposition of proximal, base-of-slope deposits was followed by an aggradational basin-fill phase of sedimentation. With the waning of tectonic activity and the filling of the fault-bounded basin, depositon of shallow-marine sands resumed (the upper sandstone unit). The stratigraphic relations of the Hamill Group in the northern Selkirk Mountains are considered direct evidence for an episode of latest Proterozoic – Early Cambrian extensional tectonism. The evidence for an episode of rift-related tectonism in the northern Selkirk Mountains supports inferences concerning the timing of this event as derived from tectonic subsidence analyses of post-rift strata of the Cordilleran miogeocline.

The behaviour of subaerially produced pyroclastic flows in a subaqueous environment: Evidence from the Roseau eruption, Dominica, West Indies

The Roseau tephra (30,000 yrs B.P.) with its source on the island of Dominica represents the largest eruption in the Caribbean in the last 200,000 years. Pyroclastic flows generated by the eruption flowed offshore into the Grenada Basin giving rise to a widespread layer of sediment gravity-flow deposits. On land the flow deposits are valley-ponded and their granulometric characteristics allow them to be classified as weakly fines-depleted ignimbrites. Offshore, the characteristics of the sediment gravity-flow deposits are consistent with deposition by numerous small-scale events rather than a single large-scale event. A comparison of the on-land and offshore deposits reveals an increase in the crystal:lithic ratio in the latter. This change can be best explained by the fragmentation of hot pyroclastic material as the subaerial flows mixed with water. Mixing and fragmentation occurred not at the air-water interface, but as flows descended the steep submarine slopes of the island of Dominica.

Climatic control of sedimentation in bays and fjords of the northern Antarctic Peninsula

The northern Antarctic Peninsula is dominated by strong climatic gradients, which are locally enhanced by subsidiary orographic and topographic effects. These gradients are reflected in the sediments of the bays and fjords of the region. The climate of the South Shetland Islands is relatively warm, with high levels of precipitation. This leads to a temperate glacial setting in which many glaciers terminate on land, and bay waters which are turbid with terrigenous debris. Sediments, which consist of terrigenous muds and sands with a low ice-rafted debris (IRD) content, are relatively thick (≈ 70 m) and have now filled in and almost completely mask the underlying basement topography. The Palmer Archipelago is colder than the South Shetland Islands, and is the wettest area of the Antarctic. It is heavily glaciated, and all major glaciers are grounded below sea level. Only rarely have sediment plumes been seen. The sediments, which are primarily terrigenous muds and sands with a variable IRD component, are thin (maximum ≈ 40 m) but are ponding in every available bathymetric depression. The pristine nature of the ice visible above sea level indicates that terrigenous debris is entering the bays from below sea level. Sediment gravity flows are important in redistributing sediment. The Danco Coast lies within the precipitation shadow of the Graham Land Plateau. Its climate is slightly colder, and is the driest in the study area, with less than half of the precipitation of the Palmer Archipelago. Nevertheless, it is also heavily glaciated. With few exceptions, sediments here are diatomaceous muds and oozes. They are very thin and drape across the rugged basement topography, rather than ponding in lows. Sediment gravity flow deposits are rare. These observations indicate a lower rate of production of terrigenous sediment here, a consequence of the drier climatic setting.

Glacigenic Characteristics of the Luoquan Formation and Sediment Gravity Flow Reworking on It

Abstract The origin of the Luoquan Formation which occurs along the southern margin of the North China Block has long been argued. Based on recent work, the Formation is considered as a glacial sedimentary sequence partially reworked by sediment gravity flow. The major evidence for the glacigene of Luoquan Formation diamictites is as follows: 1. a striated and polished pavement with various features resulting from glacial abrasion and plucking, such as crescentic gouge, crescentic fracture, streamlined form and glaciated step; 2. unsorted diamictites with striated clast, faceted clast and iron‐shaped stone formed by glaciation; 3. rhythmite with dropstones; 4. a glacial sedimentary sequence bearing advance‐retreat cycles; and 5. wide distribution of the diamictites. Glacial deposits can be distinguished from sediment gravity flow deposits by the features mentioned above. Some characteristics of sediment gravity flow existing in the Luoquan Formation diamictites indicate that glacial deposits might have been partially reworked by sediment gravity flow. Therefore, this paper suggests that the Luoquan Formation diamictite is a result of a glacial event rather than a mud flow deposit. The primary tillites are the principal contribution of the Luoquan Formation, while sediment gravity flow deposits are the redeposited diamictites and should be termed as glacigenic sediment gravity flow deposits.

Subaqueous outwash deposits in the Ingraham ridge, Chazy, New York

A 1.5 km segment of Ingraham ridge, a stratified sand and gravel ridge in northeastern New York, is composed dominantly of subaqueous outwash deposits. The core of the ridge segment may include subordinate subglacial tunnel deposits. The subaqueous outwash is composed mainly of interbedded sediment gravity flow, bar, dune, and ripple deposits that occur in channels. Sheetlike cross-stratified and cross-laminated deposits also are common, particularly on the distal western margin of the ridge segment. The ridge segment is draped by glaciolacustrine laminated sand, silt, and clay and glaciomarine massive mud. Westward decrease in grain size and scale of sedimentary structures is attributed to confining ice walls of an asymmetric embayment. Sediments are deformed in part by the melting of adjacent and buried ice. The ridge was modified during Champlain Sea regression and the reworked deposits were colonized by shallow-water organisms. Reworking may have been minor, and the broad, low morphology of the ridge may be due to subaqueous outwash deposition along its full length during retreat of the ice margin.

Lower Tertiary transgressive sediments of the Broken River Formation, Mt Somers area, Canterbury, New Zealand

Abstract In the Mt Somers area, a transgressive sequence of Upper Cretaceous to lower Tertiary sediments (Broken River Formation) was deposited on a basement of Mesozoic siliciclastic sediments (Torlesse Supergroup) and mid-Cretaceous calcalkaline volcanics (Mt Somers Volcanics Group). As a result of late Tertiary block faulting (Kaikoura Orogeny) and erosion, the sequence is preserved only in small, disconnected outliers. In the study area, lower Tertiary sediments overlie kaolinised rhyolites and ignimbrites of the Somers Rhyolite (a formation of the Mt Somers Volcanics Group). A composite lithostratigraphic sequence was constructed from five separate localities, and, although some correlations are uncertain, it is supported by palynological data. The transgressive sequence starts with terrestrial coal measures, which locally include subaerial volcanic—pebbly sediment gravity-flow deposits, followed by several units of intertidal to subtidal clays, sands, and gravels. These show a variety of physical an...

Causes of varied sediment gravity flow types on the Alsek Prodelta, northeast Gulf of Alaska

Abstract Slope failures and subsequent mass movements have been identified in Holocene glaciomarine sediment on declivities less than 1.3° on the Alsek prodelta, Gulf of Alaska. Isolated collapse features cover less than 10 percent of a nearshore sand deposit, in water depths less than 40 m. In contrast, sediment gravity flow deposits (disintegrative failures) cover more than 95 percent of a clayey silt deposit that is located in water depths between 35 m and 80 m. The morphology of individual disintegrative failures in the prodelta clayey silt indicates an eastward increase in the internal deformation and downslope translation of the failed sediment mass, the most extreme deformations being relatively large linear depressions up to 6‐m deep, 400‐m wide, and 1800‐m long, extending downslope in the easternmost part of the study area. In‐place cone penetration tests show that the nearshore sand is dense and is probably not highly susceptible to cyclic strength degradation and ultimate slope failure. The iso...

Stratigraphic Framework, Sedimentology and Structural Setting of Miocene-Pliocene Sediments of the Abu Qir Area, Offshore Nile Delta: ABSTRACT

The offshore Nile delta contains significant gas resources which are currently undergoing both production and appraisal in the Abu Qir field and adjacent areas. The principal reservoir occurs in sandstones of the upper Abu Madi Member (upper Miocene), but excellent potential reservoirs exist in the lower Abu Madi Member (upper Miocene), Sidi Salem (middle-upper Miocene), and Moghra (middle Miocene) Formations. Gas is trapped in fault-bounded anticlinal structures sealed by shales of the Pliocene Kafr el Sheikh Formation. Early wells in the Abu Qir field revealed an apparently simple stratigraphic picture. More recently, however, drilled wells have revealed a more complex stratigraphic situation. Integrated biostratigraphic and sedimentologic studies demonstrate the existence of a fault block or horst structure beneath part of the field, giving rise to the observed stratigraphic complexity. Such a structure was not visible on seismic data due to inhibited reflection quality beneath the overlying Messinian anhydrite but has since been demonstrated by reprocessing and reinterpreting seismic data. The presence of a structural style of this type provides a new concept for hydrocarbon exploration in the area. Reservoir sandstones of the Abu Madi, Sidi Salem, and Moghra Formations are interpreted as sediment gravity flow deposits whose distribution is stronglymore » influenced by syndepositional tectonics. This interpretation, supported by microfossil assemblages, provides important constraints for both reservoir modeling and further exploration.« less

Shallow-water sediment gravity flow deposits in a proximal glaciomarine sequence, Forth Approaches, North Sea

Middle Pleistocene proximal glaciomarine sediments, recovered in a borehole from the Forth Approaches, include sediment gravity flow deposits interbedded with suspension deposits and ice-rafted detritus. At least two separate flows have been identified; the lower flow (Bed A) is interpreted as the deposit of a gravelly high-density turbidity current or a density modified grain flow, while the upper flow (Bed B) was deposited from a low-density turbidity current. Sedimentation probably occurred on the mid-to-outer part of a subaquatic outwash fan. The association of mass flow and suspension sediments highlights fluctuations in the energy of this ice-proximal setting.

Natural Constraints on Exploring Antarctica's Continental Margin, Existing Geophysical and Geological Data Basis, and Proposed Drilling Program: ABSTRACT

There have been a number of multichannel seismic reflection and seismic refraction surveys of the Antarctic continental shelf. While glacial erosion has left acoustic basement exposed on portions of the inner shelf, thick sedimentary sequences occur on the passive margin of east Antarctica. The thickness and age of these strata vary due to different breakup histories of the margin. Several sedimentary basins have been identified. Most are rift basins formed during the early stages of Antarctica's separation from other Gondwana continents and plateaus. The west Antarctic continental shelf is extensive, being approximately twice the size of the Gulf of Mexico shelf. It has been poorly surveyed to date, owing mainly to its perennial sea ice cover. Gradual subduction of the spreading center from south to north along the margin resulted in old active margin sequences being buried beneath passive margin sequences. The latter should increase in thickness from north to south along the margin although no data bear this out. Hydrocarbon potential on the northern portion of the west Antarctic margin is considered low due to a probable lack of reservoir rocks. Establishment of ice sheets on Antarctica caused destruction of land vegetation and greatly restricted siliciclastic sand-producing environments. Somore » only sedimentary basins which contain pre-early Miocene deposits have good hydrocarbon prospectivity. The Antarctic continental shelf is the deepest in the world, averaging 500 m and in places being more than a kilometer deep. The shelf has been left rugged by glacial erosion and is therefore prone to sediment mass movement. Widespread sediment gravity flow deposits attest to this. The shelf is covered with sea ice most of the year and in a few areas throughout the year. Icebergs, drift freely in the deep waters of the shelf; drift speeds of 1 to 2.5 km/year are not uncommon.« less

Depositional Characteristics of Middle to Upper Miocene Point Fermin Submarine Fan, Palos Verdes, California: ABSTRACT

The Point Fermin submarine fan lies within the upper Monterey Formation of the Palos Verdes Peninsula, California. Exposures of sediment gravity flow deposits extend from the upper portion of the tuffaceous lithofacies of the Altamira shale member to the middle of the Valmonte Diatomite. Outstanding sea-cliff exposures display a three-dimensional view of a large (approximately 1000 ft across) backfilled channel, scoured into thin-bedded shale and sandstone deposits. Channel backfill deposits primarily consist of Catalina schist-bearing sandstone and breccia, and lenses of disorganized intraformational breccia and conglomerate. The exposures exhibit an overall thinning and fining-upward sequence. The coarse-grained basal portion of the sequence displays sedimentary structures and bedding characteristics which are indicative of mass deposition by extremely concentrated, viscous dispersions. Channelized breccia beds contain schist clasts up to 9 ft in diameter, and intraformation breccia lenses contain rip-up clasts of Monterey Shale up to 30 ft long. Inferred flow mechanisms are a combination of grain flow, high-density turbulent flow, and sandy debris flow. The coarseness of these deposits is indicative of a proximal source. Higher in the sequence, sedimentary structures in upper portions of individual beds are characteristic of low-density turbulent flows. This upward change in sedimentary structures within individual bedsmore » is indicative of the passage of a high-density gravity flow and the subsequent waning, low-density turbulent flow conditions which follow. Directional features indicate a mean southeasterly flow direction. These results (which contradict data from previous published reports), in conjunction with the overall coarseness of these deposits, suggest the source to be the Palos Verde uplift.« less

Carbonate apron models: Alternatives to the submarine fan model for paleoenvironmental analysis and hydrocarbon exploration

Sediment gravity flow deposition along the deep-water flanks of carbonate platforms typically does not produce submarine fans. Rather, wedge-shaped carbonate aprons develop parallel to the adjacent shelf/slope break. The major difference between submarine fans and carbonate aprons is a point source with channelized sedimentation on fans, versus a line source with sheet-flow sedimentation on aprons. Two types of carbonate aprons may develop. Along relatively gentle (< 4°) platform-margin slopes, aprons form immediately adjacent to the shallow-water platform and are referred to as carbonate slope aprons. Along relatively steep (4–15°) platform margin slopes, redeposited limestones accumulate in a base-of-slope setting, by-passing an upper slope via a multitude of small submarine canyons, and are referred to as carbonate base-of-slope aprons. Both apron types are further subdivided into inner and outer facies belts. Inner apron sediments consist of thick, mud-supported conglomerates and megabreccias (Facies F) as well as thick, coarse-grained turbidites (Facies A) interbedded with subordinate amounts of fine-grained, peri-platform ooze (Facies G). Outer apron sediments consist of thinner, grain-supported conglomerates and turbidites (Facies A) as well as classical turbidites (Facies C) with recognizable Bouma divisions, interbedded with approximately equal proportions of peri-platform ooze (Facies G). Seaward, aprons grade laterally into basinal facies of thin, base-cut-out carbonate turbidites (Facies D) that are subordinate to peri-platform oozes (Facies G). Carbonate base-of-slope aprons grade shelfward into an upper slope facies of fine-grained peri-platform ooze (Facies G) cut by numerous small canyons that are filled with coarse debris, as well as intraformational truncation surfaces which result from submarine sliding. In contrast, slope aprons grade shelfward immediately into shoal-water, platform-margin facies without an intervening by-pass slope. The two carbonate apron models presented here offer alternatives to the submarine-fan model for paleoenvironmental analysis and hydrocarbon exploration for mass-transported carbonate facies.